1. Field of the Invention
This invention relates to surface acoustic wave mercury vapor sensors, and more particularly to the in situ detection and measurement of low concentrations of gaseous mercury in air using such surface acoustic wave mercury vapor sensors.
2. Description of the Related Art
Mercury, for example, originating from nuclear fuel and weapons production and disposal, fossil fuel combustion, incineration, and industrial processes, is a major environmental pollutant that exists in air, soil and groundwater. Mercury is particularly dangerous since it can bio-accumulate within the food chain and lead to irreversible neurological disorders and other health related problems.
Current techniques that are used to detect mercury require a variety of elaborate separation strategies in conjunction with chromatographic, electrochemical, or spectroscopic methods, such as atomic absorption and emission.
German Patent No. DD 268 530 A1 is an example of current techniques and is generally directed to a method and device for the concentration determination of mercury in gases, and more specifically relates to the quantitative mercury amount determination of gases, as they arise from the Hg.sup.0 emission in reduction-ventilation-processes.
The mercury containing gas is brought into contact with a SAW (surface acoustic wave) device. The surface structure of the SAW device is designed such that the surface acoustic wave passes through a gold-plated contact zone on which the mercury is adsorbed as amalgam. Depending on the amalgam content, the propagation velocity of the surface wave changes. This velocity change is a measure for the mercury concentration and is recorded. The regeneration of the SAW device is done by thermal desorption.
From the literature, various other methods for mercury analysis are know, like photometry, fluorescence-, X-ray-, and mass-spectroscopy. Some of these methods are very costly.
Most common is the method of atom absorption (AAS) based on the cold vapor principle. The detection limit of this method is about 0.1 ng Hg/ml. With the usual dimensions of kuvettes, a relatively large sample amount is necessary for this detection limit, where an enrichment by means of a gold trap may be carried out to improve the sensitivity. Here, in order for the analysis, the mercury has to be released again, where thermodynamic and hydrodynamic problems occur, which is disadvantageous.
In the "American Industrial Hygiene Association Journal", 1975, p. 897-901, a mercury detection method is described based on a piezoelectric crystal. Here, the mass loading of a gold-plated longitudinal oscillator is detected as a change in the resonance frequency.
Conventional use of a bulk oscillator limits the sensitivity because of a relatively low operation frequency, since for a given mass of the layer the dynamic mechanical tension on the crystal surfaces is frequency dependent.
Although these techniques may be able to detect mercury, they are not as advantageous as the surface acoustic wave mercury vapor sensors of the present invention, at least in part because they are not appropriate for in situ monitoring, and typically, samples must be collected and shipped to a central processing facility for analysis.
The numbers in brackets below refer to references listed in the Appendix, the teachings of which are hereby incorporated by reference.
Those concerned with these and other problems recognize the need for an improved surface acoustic wave mercury vapor sensors.